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Image of the Week

First Gaia BP/RP deblended spectra

DSS coloured image of the double star HD270801. Bottom left: the observed BP spectrum in black and the two extracted spectra in magenta and blue; in red the extraction residuals. Bottom right: the same for RP

To test the photometric deblending software, which is to be fully deployed at the Cambridge Data Processing Centre by the end of 2015, a set of known bright double stars has been processed. The figure above shows the output of the code applied to the BP/RP spectra of the double star HD270801, the first Gaia BP/RP deblended object. HD270801 is a ~10th magnitude known double star with a separation of 0.68 arcsec (~12 along-scan pixels), which means that the components are well separated in the AF field of view. In the photometers, however, the spectra overlap significantly.

Deblending BP/RP spectra in crowded fields is a real challenge. The software, developed within the DPAC-CU5 Gaia pipeline, minimizes the weighted sum of square deviations between the observed spectrum and the theoretical one, using a principal components representation of the spectral shapes. The search for the minimum of the square deviation is performed through a non-linear least squares technique, based on a version of the Levenberg-Marquardt algorithm.

Considering the along-scan dispersion, the superposition of two field of views, and the large stellar density in many regions of the sky, stellar spectra often overlap. In order to obtain accurate values for the astrophysical parameters and to compensate for the chromaticity shift, the BP/RP flux extraction of blended transits (i.e. observations of overlapped spectra) is mandatory: an error in the extraction could induce a variation of the spectral morphology and produce errors in the estimation of parameters. Moreover, it could also affect the astrometric performances.